Help - Identification Required

B and I went to 'Warham Camp' this morning. This Iron Age Fort was built over 2000 years ago next to the River Stiffkey by the local tribe, the Iceni.  The structure is still quite obvious and one can walk around the outer and inner walls where lots of wild flowers bloom.  It is chalkland and so is frequented by the beautiful Chalkhill Blue butterfly.  This is a photo of the female which is mostly brown (see the extra for a photo of it with its wings spread).  They are more secretive than the males so harder to spot.
I noticed when I was processing the photo that there is a caterpillar on the knapweed in the above photo and I'm unable to identify it.  It doesn't appear to belong to the chalkhill blue and despite a search on the internet I'm still none the wiser. 

Does anyone know what it is?